To address the dual challenges of achieving high-speed transmission and ensuring smooth evolution in passive optical networks (PONs),this paper proposes a seamless upgrade strategy from current PONs to future high-speed systems using multi-ary differential phase shift keying (DPSK).The newly introduced PON signal employs multi-ary DPSK modulation,which nearly eliminates the crosstalk from the new DPSK-modulated signal to the coexisting legacy amplitude-shift keying (ASK) modulated PON signal.By leveraging multi-ary modulation,the new PON signal achieves a high bit rate even with commercially available bandwidth-constrained transceivers.Wavelength division multiplexing (WDM) is further applied to multiply the downstream bandwidth of the new PON.Since the DPSK-modulated downstream signal introduces minimal crosstalk to the remodulated ASK upstream signal, a colorless optical network unit (ONU) can be realized through upstream remodulation.Simulation results demonstrate the feasibility of both 6-ary and 8-ary DPSK based schemes,with error-free operation achieved for all signals. The proposed system shows that the new PON downstream signal induces negligible crosstalk to both the coexisting legacy PON downstream signal and the remodulated upstream signal.Consequently,the new PON signal achieves a bit rate exceeding 30 Gb/s per wavelength using current 12.5 G-class transceivers.